At the beginning of October, California's fire season was already threatening to be the worst on record. Over 600,000 acres of state land had burned by that point — a total driven by infernos like the Mendocino Complex Fire, which burned 459,000 acres to become the largest fire in California history. The total from the first nine months of 2018 alone was considerably more than the 506,000 acres that had burned on state land in all of 2017, which was itself more than twice as much as burned in 2016.

One slim hope for the rest of the year was that fall rains might keep California's trees and vegetation relatively moist. But the rains did not come. Instead, the state got sustained high winds, which dried out the already drought-stressed forests and undergrowth even further. And when a fire got going in the brush, forests, and grasslands of Butte County last week, the result was the deadliest fire in state history.

Fed by 30 to 50 mph winds, the Camp Fire grew at a spectacular rate, bearing down on the town of Paradise (population: 27,000) at high speed and blowing a dragon's plume of sparks and embers before it. The speed, the ferocious heat, and the hundreds of smaller fires catching ahead of the main blaze overwhelmed the city's defenses so quickly that despite a carefully-rehearsed evacuation plan, not everyone managed to escape.

Meanwhile, the Woolsey Fire has so far scorched over 98,000 acres in Southern California, including much of the city of Malibu, and killed at least three people.

It's a grim reminder of what climate change is going to do to the forests of the world. This type of turbo-wildfire is probably just the beginning.

Forests only cover about 30 percent of the world's land area, but they are critical to the overall health of the planet. They provide habitat for millions of animal species, help cleanse air and water, and absorb about a quarter of all the carbon dioxide emitted by humans. They are also by far the biggest source of biomass — that is, the total mass of all living things — accounting for about four-fifths of the total.

Climate change will have two contradictory effects on forests, William Anderegg, a climate scientist at the University of Utah who specializes in tree biology, told The Week. On the one hand, some of its effects will stimulate forest growth: For instance, carbon dioxide is literally food for trees, a warmer world means a longer growing season, and a warmer atmosphere means more precipitation on average. In many forests — like in the Eastern U.S. where forests that were clear-cut in ages past are slowly growing back — they are absorbing millions of tons of carbon.

But on the other hand, a warmer world would also harm forests because it means more drought, more fires, and more insect infestations. The Western U.S., for instance, has suffered all three of these problems, which also tend to interact with each other. An ongoing infestation of bark beetles has been largely caused by climate change, as drought-weakened trees struggle to fight off the insects and winters rarely get cold enough anymore to kill their larva. The beetles have devastated populations of pinon pine, lodgepole pine, ponderosa pine, Engelmann spruce, and other species from British Columbia to northern Mexico, with a death rate of over 90 percent in some areas. The insects have damaged nearly 87,000 square miles of forest in the Western U.S. (or roughly the size of Ohio and New York state put together) between 2000 and 2017, and over 70,000 square miles in British Columbia.

Other forests have been just killed outright by drought and heat. The way deciduous trees tap water from the soil is through evaporation at the leaf surface, which pulls water up through the tree's xylem (basically its arteries) through capillary action. But as temperatures increase, the "atmospheric water demand" increases non-linearly — that is, if the temperature rises by, say, 0.5 percent, the amount of water evaporating out of the leaf grows by more than 0.5 percent. As Anderegg's previous research has demonstrated, the combination of heat and dryness can kill trees by dropping water pressure in the xylem so low that the water changes from a fluid to a gas, forming a bubble that blocks off part of the passage. Block too much of the xylem, and the tree dies.

Yet other forests have simply burned down in increasingly apocalyptic wildfires — especially ones parched by drought. As we have seen, California's 2018 fire season is now the worst on record, beating the 2008 record of 1.4 million acres burned by 150,000 and counting. In Colorado, all but one of the 20 largest wildfires in the state's recorded history have happened since 2002 (a notoriously bad drought year), and five happened in 2018 alone. In 2015, five million acres burned in Alaska. In 2012, some 70 million burned in Russia. Europe has seen similarly apocalyptic fire seasons (and beetle outbreaks) over the past few years.

Under such stresses, the increased CO2 food supply makes little difference, just as how a man with nothing to drink isn't helped much by an endless supply of protein powder.

Whether the positive or negative effects of climate change will predominate in Earth's forests is, of course, a terrifically complicated question. Trees are large and takes decades or more to grow to maturity, making experiments expensive and thus much rarer than they should be. Nevertheless, climate scientists are constructing a convincing circumstantial case for how climate change is liable to affect forests. It's pretty ominous. "The latest evidence suggests that the negative effects are starting to win out, and that will likely get worse in the future," says Anderegg.

Not all forests will be equally affected, however.

There are about 360 billion metric tons of carbon contained aboveground in all the world's forests. Broken down between different types of forests, roughly 72 percent of that mass is contained in tropical forests (like the Amazon basin), 13 percent in temperate forests (like in California and most of the United States), and 15 percent in boreal forests (as in northern Canada and Siberia).

Tropical forests are considered to be at the lowest risk from the effects of global warming. They're located around the equator, which has if anything warmed a bit slower than average. They have tremendous diversity of tree species, which should make them a bit more resilient to pests. And they are also (usually) very wet, and so somewhat less vulnerable to wildfires. (Of course, this doesn't mean they're invulnerable to climate change; recentpapers suggest we might even be overestimating their resilience.)

Despite what's happening in the American West, temperate forests are also not at the highest risk. While many individual tree species and areas are under severe threat, the category as a whole is reasonably stable and even increasing its carbon uptake for now, thanks partly to expanding forests in Europe and China.

The forests most immediately vulnerable to climate change are probably the boreal forests — the vast belt of hardy evergreen woodlands stretching from northern Quebec up through northern Alaska, and from eastern Siberia to Finland and Norway. Such forests are drier than tropical forests and have vastly less species diversity, partly because more than half are managed in some way. This tends to mean cultivating just a few species for timber or paper, making them more vulnerable to pests. They also have the disadvantage of less light in their extreme northerly locations, making it harder for them to take advantage of increased CO2 supply. Finally and most importantly, this region is warming between roughly 1.5 to two times as fast as the planetary average, and has thus been experiencing severe ecological disruption.

It's thus quite easy to imagine a sort of perfect storm obliterating huge swathes of the boreal forest in the relatively near future. Drought and accelerating warming could create ripe conditions for pestilential beetle outbreaks, which could then worsen drought and warming and create a tinderbox of dead wood for gargantuan wildfires.

But there are two background facts that make the situation even more alarming: First, there is the simple reality that forests can be destroyed very quickly and take many years to grow back — and if conditions are bad enough, the entire biome can change away from forests altogether. Second, the gigantic amount of carbon contained under boreal forests in peat, permafrost, and soils, is estimated to match or even exceed the amount contained aboveground in tropical forests.

The implications of that last fact, in particular, are pretty frightening.

Climate change is happening because human beings are burning fossil fuels, making cement, changing forests to farmland, and so on, thus releasing carbon dioxide into the atmosphere. This traps heat by reflecting infrared radiation from sunlight-warmed ground back into the Earth. In recent years, this carbon has amounted to about 10 billion metric tons (leaving out the oxygen atoms for the sake of easy comparison to non-burned carbon).

But that figure is completely dwarfed by the staggering volume of carbon locked up in boreal forest lands and tropical forests, which is on the order of 600 billion metric tons, or 60 times what humans have recently emitted. If forests simply become net neutral in carbon terms, that would effectively increase emissions by a third. But sustained degradation of the world's forests could make them a net source of carbon — and just 1 percent of that total would be more than half a year's total current human emissions.

Faster releases are the stuff of nightmares. To return to the boreal forests, one of the biggest sources of carbon there is methane (which is a far stronger greenhouse gas than carbon dioxide) locked in the permafrost they grow in. If just 10 percent of that were to escape, it would be roughly equivalent to increasing the atmospheric CO2 concentration by a factor of 10 at a stroke. At that point, warming would escape human control altogether, outside of the most wild-eyed geoengineering schemes.

Republicans, of course, are well beyond useless on this problem. Outright climate change denial has become so ludicrous that many GOP elites like Sen. Marco Rubio (R-Fl.) have abandoned it, but it is nothing but a tactical retreat. On CNN, he suggested recently that all that can be done is mitigation: "No matter what we do with laws, if tomorrow we stopped all — say we went to all solar panels and did all that stuff, which is not realistic, this trend would still continue," adding that "I'm also not going to destroy our economy."

In reality, this kind of foot-dragging will unquestionably destroy thousands of American communities, kill tens if not hundreds of thousands of American citizens, and possibly even threaten the existence of the United States as an organized political entity. Paradise, California, did try to mitigate climate risks, in the form of strict building codes, a well-drilled evacuation plan, and an experienced and well-funded state fire agency. All that did not save the city's structures and it did not save the people who died in their cars trying to escape as the blaze overtook them.

Climate policy is needed not just to protect the world's forests as wild habitat for animals and a useful source of materials for businesses, it is needed to protect human society, the United States very much included.